While trastuzumab has been shown to prevent recurrences of breast cancer, some women may also experience damage to their heart muscle (including heart failure) as a result of their treatment. The investigators hope to learn if standard medications used in heart failure can prevent heart damage caused by trastuzumab in women with breast cancer. The investigators would also like to know if there are any ways to detect this damage earlier using magnetic resonance imaging (MRI) and blood tests.